SC2440 SEMTECH [Semtech Corporation], SC2440 Datasheet - Page 10

• Manufacturer Part Number: SC2440
• Description: 2.5 MHz Dual Switching Regulator with Integrated 2A Switches
• Manufacturer: SEMTECH [Semtech Corporation]

Minimum On Time Consideration
The operating duty cycle of a step-down switching
regulator with diode rectifier in continuous-conduction
mode (CCM) is given by
where V
voltage drop across the rectifying diode.
Duty cycle decreases with increasing
current-mode control, the PWM modulating ramp is the
sensed current ramp of the power switch. This current
ramp is absent unless the switch is turned on. The
intersection of this ramp with the output of the voltage
feedback error amplifier determines the switch pulse
width. The propagation delay time required to
immediately turn off the switch after it is turned on is
the minimum switch on time (T
measurement of the SC2440 with low
that the minimum on time is about 105ns at room
temperature. T
temperature coefficient (Figure 6). The power switch in
the SC2440 is either not turned on at all or for at least
T
than the minimum on time, the regulator will either skip
cycles or it will jitter.
Example: Determine the maximum operating frequency
of a dual 12V to 1.0V and 12V to 3.3V switching
regulator using the SC2440.
Assuming that V
(10% high line), the corresponding duty ratios, D
D
using (2).
Applications Information
POWER MANAGEMENT
ON (MIN)
2
2005 Semtech Corp.
, of the 1.0V and 3.3V converters can be calculated
D
D
1
. If the required switch on time (=
V
CESAT
13
IN
V
2 .
OUT
is the switch saturation voltage and V
V
1
D
. 0
D
ON (MIN)
. 0
= 0.45V, V
45
V
V
D
45
CESAT
. 0
also exhibits a slight positive
25
CESAT
. 0
11
= 0.25V and V
ON (MIN)
V
V
V
OUT
V
IN
OUT
IN
). Closed-loop
D
f
ratio. In peak
ratios shows
) is shorter
IN
= 13.2V
1
and
D
(2)
is
10
If the ambient temperature can be as high as 85 C, then
the maximum operating frequencies of the 1.0V and the
3.3V converters will be
Channel frequency should be set below 920KHz to allow
margin for load transient.
Minimum Off Time Limitation
The PWM latch in Figure 2 is reset every period by the
clock. The clock also turns off the power transistor to
refresh the bootstrap capacitor. This minimum off time
limits the attainable duty cycle of the regulator at a given
switching frequency. Measurement shows that the power
transistor needs to be turned off for at least 120ns every
switching period to properly reset the latch and to refresh
the bootstrap capacitor. For a step-down converter, D
increases with increasing
cycle is higher than the attainable maximum, then the
120
D
D
2
ns
2
Figure 6. Variation of Minimum On Time with
13
2
3 .
2 .
130
120
110
100
90
80
MHz
3
-40
3 .
Minimum On Time vs Temperature
. 0
Temperature.
45
-20
respectively. .
. 0
45
. 0
Temperature (°C)
0
25
20
V
V
OUT
IN
. 0
40
ratio. If the required duty
28
120
60
D
1
ns
80
www.semtech.com
SC2440
920
100
KHz
and